The present invention relates to an image projection apparatus, such as a projector, an image projection optical system including a projection lens that is a zoom lens, which is used for the image projection apparatus.
Image projection apparatuses (projectors) projecting light from an image-forming element such as a liquid crystal panel onto a projection surface such as a screen should satisfy the following requirements.
1. In a three-panel projector using three image-forming elements for R, G, and B, a space is required where optical elements, such as a color-combining prism combining three color light components from the three image-forming elements and a polarizing element, between the image-forming elements and a projection lens. Thus, a back focus of the projection lens needs to be long to some extent.
2. The projection lens needs to be a telecentric optical system whose image-forming-element side pupil is located at infinite distance in order to reduce an influence of incident angle dependence of an optical film, such as a polarization splitting film, and to ensure good pupil consistency with an illumination optical system illuminating the image-forming elements with light.
3. Distortion (aberration) of the projection lens needs to be corrected well in order to prevent distortion of a projected image at its contour portion with respect to an original image.
4. When three color images are combined to be projected onto the projection surface, pixels of the three colors corresponding to each other are superposed well in the entire image area on the projection surface. Thus, color displacement (chromatic aberration of magnification) generated in the projection lens needs to be corrected well in the entire visible wavelength range.
5. The apparatus should be miniaturized.
6. A larger image can be projected at a shorter projection distance.
7. The projection lens needs to have a zoom function in order to enable selection of the size of the projected image at a certain projection distance.
8. The temperature of the optical system increases because a high intensity lamp is used as a light source, so that focal shifts (focus variation or focus displacements) and performance deterioration caused by temperature changes should be prevented.
Projection lenses for projectors satisfying the above requirements have been disclosed as follows.
Japanese Patent Laid-Open No. 2001-235679 (corresponding to U.S. Pat. No. 6,633,436) has disclosed a projection lens including six lens units that have, in order from a magnification side, negative, positive, positive, negative, positive, and positive refractive powers. The projection lens moves at least two of the six lens units to vary its magnification.
Such a negative-lead type zoom lens in which the lens unit having the negative refractive power is disposed closest to the magnification side is used for many projectors because it facilitates an increase of its field angle and has advantages that a good performance can be ensured in a short-distance projection and the like. However, in the negative-lead type zoom lens, the movement amount of a movable lens unit for magnification variation is large, so that the magnification is difficult to be increased and chromatic aberration of magnification is significantly varied.
Further, a positive lens unit is often disposed closest to the magnification side to correct the distortion, which increases the size of the zoom lens. In many recent zoom lenses as disclosed in Japanese Patent Laid-Open No. 2006-84971 (corresponding to U.S. Pat. No. 7,079,324), an aspheric lens made of plastic is used to correct the distortion, which eliminates the positive lens unit closest to the magnification side and thereby reduces the size and weight of the zoom lens.
However, the refractive index of the plastic lens is more significantly varied with the temperature changes than that of a glass lens. Therefore, the refractive index of the plastic lens is significantly varied by heat generated by projecting light, which causes focus variations and performance deterioration.
Japanese Patent Laid-Open No. 2005-266103 (corresponding to U.S. Pat. No. 7,215,477) discloses a method for correcting focus variations using two aspheric lens elements respectively having positive and negative optical powers.
However, in recent projectors, the temperature of the projection lens increases significantly with increase of their brightness (intensity), so that changes of the refractive index of glass by the temperature changes cannot be negligible as well as that of plastic.
The problem is that the change amounts of the refractive index of glass materials by the temperature changes are different depending on their constituents. Specifically, the refractive index of most glass materials included in a region of an Abbe number of νd<68 (hereinafter referred to as a C-region) changes with the temperature change by a positive amount of from 2.5×10−6 to 8.5×10−6(/° C.). In contrast, the refractive index of low dispersion glass materials included in a region of 68<νd<75 (hereinafter referred to as a B-region) changes by the temperature change by an amount of −0.7×10−6(/° C.), which is near zero. Further, the refractive index of ultra-low dispersion glass materials included in a region of 75<νd (hereinafter referred to as an A-region) changes with the temperature change by an amount of −6.0×10−6(/° C.), which is an amount with opposite sign to that of the materials in the C-region.
Since positive and negative lenses are generally designed to dominantly use the materials of the C-region, the focus variations are often corrected without any intention. However, the projection lens with a long back focus includes a negative lens unit having a high optical power which is disposed closer to a reduction side than an aperture stop in order to ensure a sufficient distance to a reduction side focal plane. Therefore, a larger negative spherical aberration than usual is generated. This makes it necessary to correct the spherical aberration by using a lot of surfaces having a positive optical power.
In this case, however, a positive lens unit disposed closer to the reduction side than the aperture stop is often formed of a low-dispersion material included in the A or B region so as to prevent longitudinal chromatic aberration from being exceedingly increased.
The use of the low-dispersion material for the long-back-focus projection lens generates an additional focus variation due to a temperature rise of the negative and positive lens units, resulting in a large focus variation even when using only glass materials.